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Scientific excitement about cancer treatment tends to focus on chemistry and biology — developing drugs that are more effective and selective than traditional chemotherapy — but physics plays a role too, by enhancing the precision of radiotherapy.
Conventional radiotherapy uses high-energy X-rays to destroy cancer cells, although the side-effects are often substantial because the beam damages peripheral tissues on its way to the target. In contrast “proton beam therapy”, which is rapidly gaining popularity, delivers radiation in the form of protons (hydrogen nuclei) that can be directed much more precisely at the tumour. Protons deposit almost all their destructive energy inside the cancer.
Proton beam therapy enjoyed a moment in the UK headlines in 2014 when Brett and Naghmeh King removed their five-year-old son Ashya from Southampton general hospital, without doctors’ permission, to undergo proton treatment in Prague. The UK had no proton oncology facility, which is still the case, though five are being built or planned. The government funds two, through the National Health Service, and Wales-based Proton Partners International, a private company, three.
The NHS proton beam centres, for which the government is providing £250m, will be at University College London hospital and the Christie hospital in Manchester. Proton Partners, which has received almost £100m from the Welsh government and UK investors, is building more compact proton beam centres in Newport, the Imperial West campus in London and in Northumberland, in north east England. They will take NHS as well as private patients.
All five facilities are likely to be operational by 2020. Elsewhere in the world, 44 proton beam therapy centres are in action, says Mike Moran, Proton Partners chief executive, and many more are on the drawing board. “I think we will eventually have seven operational centres in the UK,” he adds. The leading supplier of proton therapy equipment is Ion Beam Applications, a Belgian medical technology company.
Although proton beams have been around for decades, their rapid adoption for oncology is the result of various technological advances, Mr Moran says: “The really significant change is that 3D imaging allows us to put the protons on to a tightly defined spot on the body.”
Treatment centres are inevitably expensive to set up, costing $40m or more depending on size and complexity. The protons have to be accelerated to the required energy in a cyclotron. Electromagnets focus the protons towards a “gantry” which rotates the beam around the patient to achieve the best angle for treatment. A nozzle delivers the controlled beam to the targeted tumour.
Oncologists estimate that between 10 per cent and 20 per cent of patients treated with conventional radiotherapy would do better with proton beams. The main advantage is that their energy and direction can be controlled more tightly than X-rays. This makes them very useful for treating adult tumours — close to vital organs such as brain, spine and lungs — and a wider range of childhood cancers in order to spare developing healthy tissues from irradiation.
In January Massachusetts general hospital researchers published clinical evidence in Lancet Oncology of the superiority of proton therapy when treating medulloblastoma, the most common brain cancer in children. The seven-year study showed protons to be as effective as standard X-ray therapy in getting rid of a tumour while inflicting less long-term collateral damage.
“Our results indicate that proton therapy maintains excellent cure rates in paediatric medulloblastoma while reducing long-term side effects, particularly in hearing and neurocognitive function, and eliminating cardiac, pulmonary, gastrointestinal and reproductive effects,” says Torunn Yock, lead author. Particularly for the youngest children, he adds, “proton therapy can make a big difference to their lives.”
Some oncologists believe therapy with carbon ions — 12 times heavier than protons — could produce even better results in some tumours. The high cost of equipment to generate carbon ion beams and a lack of clinical evidence for their superiority to protons mean this field is in its infancy. A handful of centres are operating, mainly in Asia.
Technology is improving conventional X-ray therapy, too. “Radiotherapy is seeing a number of technical developments which have the potential to bring about a step change in what we can achieve,” says Gary Royle, medical radiation physics professor at University College London.
Radiotherapy is “not always used to its full potential”, adds Ricky Sharma, UCL professor of radiation oncology. UCL aims to move from X-ray systems that usually deliver an even dose of radiation across tumours. New imaging techniques and biopsies will identify the most intractable part of the cancer in order to target the radiation there.
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